This invention relates to microwave filters and in particular to an improved tunable microwave notch filter.
In various radar systems interleaving series of pulses are used such that the series is made up of two or more pulse trains in which each pulse train has a common carrier frequency. It is then of interest to provide circuits in the radar transmitter which will eliminate, or "notch out" one carrier frequency when in the presence of another. Typically, these microwave carriers will differ by 10's of megahertz. Thus, use of conventional filter circuits is rendered difficult due to the very high Q factors and steep rolloffs needed. Also, if one or more of these carriers is desirable of adjustment, then the filters must also be adjustable, usually electronically. Finally, to eliminate interference, one typically seeks 50-60 or better isolation, along with fractional dB insertion loss to the transmitted signal.
These functions have recently been successfully provided by the device disclosed in U.S. Pat. No. 3,895,304 by G. I. Klein entitled TUNABLE MICROWAVE NOTICH FILTER. The Klein filter is comprised of two filter stages and tuning (for notch location) is accomplished by phase shifting in each stage. Although this notch filter represents a substantial improvement over prior art devices it is a component of substantial volume and complexity and the two filter stages must be precisely tuned to have their notches line up in the frequency domain. Accordingly, it is desirable that a reduction in size and complexity and the elimination of the necessity of the fine-tune phase notch of the two phase shifters in this type of filter be realized. The present invention is directed toward providing a notch filter that meets these requirements.